An easily reproducible SMS exploit was disclosed and demonstrated today at the 25th Chaos Communication Congress (25C3). The exploit is effective against a wide range of Symbian S60 smartphones and will effectively prohibit victims from receiving SMS messages.

The Chaos Communication Congress is a popular event among international "hacker" enthusiasts. It has been organized by the Chaos Computer Club since 1984, has been held in Berlin since 1998 and typically takes place between December 27th and 30th.

Today's Security Nightmares 2009 presentation included a demonstration of the Curse of Silence exploit, which was researched by Tobias Engel of the CCC.

According to Engel's research, the exploit affects the messaging components of Nokia Series 60 phone versions 2.6, 2.8, 3.0, and 3.1. Our own tests determined that Sony Ericsson UiQ devices are vulnerable as well.

According to Engel's research, the vulnerable phones fall into two camps: S60 versions 2.6/3.0 (2FP2/3) and versions 2.8/3.1 (2FP3/3FP1). That's still too many numbers, so let's just select two phones.

The vulnerability is very simple to exploit via an SMS message. No special software is required and the message can be drafted from a large number of phones. The message just needs to be formatted in a particular way. (We will not provide exact details here.)

What happens when a vulnerable phone receives the exploit message?

Example 1 — on the older 6680 nothing happens. Nothing at all… The first exploit message is enough to crash the SMS messaging service. It is a completely silent attack and there are no hints of trouble presented to the victim. The phone will simply stop receiving SMS (as well as MMS) messages.

Example 2 — on the newer N95, nothing will happen until several messages have been sent by the attacker. Then, once the critical limit has been reached, the phone will prompt an alert: "Not enough memory to receive message(s). Delete some data first."

The attack messages will not be visible from the Inbox, and deleting previously received messages will not resolve the problem.

There will also be one additional notification on the N95. A blinking envelope, indicating that the Inbox is full, appears in the upper right-hand corner of the display.

Turning the N95 off and on again may return some limited functionality, but that functionality is very fragile. One multi-part message was enough to completely disable our test phone's SMS/MMS service, at which point even cycling the power did not help.

Exploited phones will remain otherwise completely functional; only the SMS/MMS messaging is affected. Practically speaking, this also means no SMS notifications of voicemail, though the phone log will display the missed call.

A firmware fix is not yet available. Performing a hard-reset is the only manual solution. And backing up the phone also backs up the exploit messages and the damaged messaging service.

Shameless self-promotion begins:

However — Engel practiced reasonable disclosure, which is why we have had time to test the exploit ourselves before today's CCC demonstration. Our Mobile Security solution will detect the exploit and can repair affected phones.

The exploit is detected as Exploit:SymbOS/SMSCurse and Mobile Security is capable of repairing exploited phones so that it will not lose any messages. Messages that have been sent while the messaging service is jammed will of course be lost.

Hopefully this exploit will not be widely used. We don't see much of a profit motive after all. Still, there were thousands of participants at this year's CCC and many of them saw the demonstration. As easy as it is to utilize the Curse of Silence, someone will surely try this for harassment…

A free seven day trial of Mobile Security can be directly download to phones from here.

We will have a video demonstration available soon.Update: Info on the video is here.